The present invention relates to a method for transmitting at least one first and one second data signal in polarization division multiplex in an optical transmission system which includes a transmitting arrangement and a receiving arrangement which are connected to one another via at least one optical fiber link section.
In optical transmission systems, the transmission capacity of existing optical transmission systems can be extended by transmitting the optical data signals in polarization division multiplex. To transmit optical data signals in polarization division multiplex, in each case two carrier signals which are respectively modulated with a data signal are generated in at least one transmitting arrangement with the same wavelength. The first and second modulated signals exhibit a mutually orthogonal polarization. The mutually orthogonally polarized modulated signals are combined to form an optical polarization division multiplex signal. The optical polarization division multiplex signal is injected into the optical transmission fiber and transmitted via the optical transmission link to a receiving unit. At the receiving end, the two orthogonally polarized modulated signals are recovered from the polarization division multiplex signal in a wavelength-dependent and polarization-dependent manner.
One of the essential problems in transmitting optical data signals in polarization division multiplex is the recovery of the two orthogonally polarized modulated signals from the polarization division multiplex signal.
This requires the determination of a control criterion for controlling a polarization transformer arranged at the receiving end from the transmitted optical polarization division multiplex signal. The modulated signals transmitted polarized orthogonally with respect to one another are separated with the aid of the polarization transformer controlled via the suitable control criterion and, for example, a downstream polarization splitter or a polarization filter.
Different control criteria are known for controlling the separation of the two orthogonally polarized signals at the receiving end. From the publication xe2x80x9coptical polarization division multiplexing at 4GB/Sxe2x80x9d by Paul M. Hill et al., IEEE Photonics Technology Letters, Vol. 4, No. 5, May 1992, the use of coherent techniques in combination with pilot tones for reconstructing or, respectively, separating the polarization division multiplexed optical signals is known. Furthermore, separating the polarization division multiplexed optical data signals via a correlation signal generated from the recovered clock and from the received optical signals is known from the publication xe2x80x9cFast Automatic Polarization Control Systemxe2x80x9d, Heismann and Whalen, IEEE Photonics Technology Letters, Vol. 4, No. 5, May 1992.
Moreover, a method for separating the two signals transmitted orthogonally polarized at the receiving end via the evaluation of the autocorrelation function of one of the signals transmitted is known from international application WO 01/65754 A1. In this arrangement, the control of the polarization control element at the receiving end is determined with the aid of the extinction ratio of the respective transmitted signal, calculated via the autocorrelation amount.
An object of the present invention is to devise a novel method and optical transmission system for transmitting high-bit-rate optical signals in polarization division multiplex.
A key advantage of the method according to the present invention can be seen in the fact that to transmit at least one first and one second data signal in polarization division multiplex in an optical transmission system, in a first step, a first modulated signal is generated at the transmitting end by modulating a first carrier signal with the first data signal and a second modulated signal is generated by modulating a second carrier signal, which differs from the first carrier signal by a differential frequency, with the second data signal. In a second step, the first and second modulated signals are polarized orthogonally with respect to one another and combined to form an optical multiplex signal and are transmitted. In a third step, at the receiving end, the optical multiplex signal is conducted via a polarization control element to a polarization splitter which splits the optical multiplex signal into the first and second modulated signal. In a fourth step, the first modulated signal is converted into a first electrical signal and the second modulated signal is converted into a second electrical signal. In a fifth step, the spectral component of the first and/or of the second electrical signal at the differential frequency is determined and from this at least one control signal is derived for controlling the polarization control element. Using the method according to the present invention, at least one polarization control element arranged at the receiving end is controlled particularly advantageously for the precise separation of a first and second modulated signal transmitted in polarization division multiplex at the receiving end. For this purpose, the squaring characteristic of an optoelectrical transducer, such as a photodiode, is used. Due to these squaring characteristics, unwanted spectral components arise at the differential frequency in the electrical spectrum of the electrical signal output at the output of the optoelectrical transducer if the separation of the two modulated signals transmitted in polarization division multiplex, performed with the aid of the polarization splitter, is not precise. These spectral components located at the differential frequency arise both in the first electrical signal and in the second electrical signal. According to the present invention, the amplitude of these spectral components is evaluated for forming at least one control signal for controlling the polarization control element. During this process, the polarization control element is controlled, such as with the aid of the at least one control signal, in such a manner that the spectral component arising at the differential frequency becomes minimal. Such a severe control criterion makes it possible to separate the two modulated signals transmitted in polarization division multiplex as precisely as possible at the receiving end.
The first or second modulated signal is advantageously delayed at the transmitting end which results in effective decorrelation of the first and second modulated signal. This makes it possible, additionally, to increase the severity of the control criterion.
A further advantage of the present invention can be seen in the fact that, to distinguish between the first and second electrical signal, at least one pilot tone signal is superimposed on the first and/or the second carrier signal at the transmitting end. Advantageously, a pilot tone having a specified frequency, with the aid of which, after the first and second modulated signal have been separated with the aid of the polarization splitter and converted into a first and second electrical signal at the transmitting end, an unambiguous identification of the first and second electrical signal as such becomes possible, is superimposed on the first and/or second modulated signal. As an alternative, the first and second data signal can be transmitted with different transmission bit rate for distinguishing the first and second electrical signal. In an alternative embodiment, the respective electrical signal is advantageously identified via its current transmission bit rate.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.